Voice of the Dragon: the rotating corrugated resonator

A simple, yet unusual, child's toy illustrates some basic features of the physics of resonance, waves and fluid dynamics. The acoustic modes of a corrugated tube open at both ends and rotating in a plane were examined as a function of rotational frequency and found to be similar but not identical to those of a stationary open-ended organ pipe. Measurements of the pressure difference at resonance across the tube ends as a function of rotational frequency agree well with a simple analysis based on Bernoulli's principle. A mechanism of sound production is proposed whereby the tube resonantly amplifies acoustic perturbations to the axial air flow, engendered by the corrugations, that occur at frequencies equal to the resonant frequencies of the tube. The mechanism is supported by direct measurements of the axial air speed as a function of rotational frequency.